Add new option and predefined macros for OpenCL C 3.0 version.
LGTM! Thanks! Even though I don't imagine the discussion on the RFC (http://lists.llvm.org/pipermail/cfe-dev/2020-September/066883.html) will affect this functionality I think it's good to give some extra time to react when starting the implementation of such big features.
It is probably better to commit this next week.